Zobel's work on filter design was revolutionary and led, in conjunction with the work of John R . Carson, to significant commercial advances for AT & T in the field of frequency division multiplex ( FDM ) telephone transmissions.
2.
Trunk circuitry hearing a 2, 600 Hz tone for a certain duration would go idle . ( The duration requirement reduced falsing . ) Some systems used tone frequencies over 3, 000 Hz, particularly on SSB frequency division multiplex microwave radio relays.
3.
Starting in the early 1950s, frequency division multiplex was used to send up to 5, 400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the " hop " to the next site, up to 70 km away.
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